This invention relates to a method of making microcapsules containing hydrophobic capsule core material and more particularly to a method of making microcapsules having an improved capsule core material retainability.
Microcapsules find their usefulness in various fields such as pressure-sensitive copying papers, heat-sensitive recording papers, adhesives, fertilizers, pharmaceuticals, foods, cosmetics, etc. There are known various method for making microcapsules useful for those purposes. Among them, there are included the "coacervation" method, the "interfacial polycondensation" method and the "in situ polymerization" method. It is also known that microcapsules each having a capsule wall of an aminoaldehyde resin are superior in the water resistance and the solvent resistance. For example, U.S. Pat. No. 3,016,308 discloses a method of making microcapsules in which urea-formaldehyde polycondensation resin capsule walls are formed in the presence of carboxymethyl cellulose. U.S. Pat. No. 3,516,941 discloses another method of making microcapsules in which urea-formaldehyde polycondensation resin capsule walls are formed in a suspension substantially containing no wetting agent. In these conventional methods of making microcapsules utilizing urea-formaldehyde resin, however, the system for making microcapsules must be carefully controlled as by stepwise and continual dilution since effective deposition of the polycondensation product on the capsule core material cannot be expected. In order to effectively carry out deposition of the polymerized resin on the capsule core material, some attempts have been made. For example, Japanese patent publication No. 12,380 of 1962 proposes to utilize as a dispersing agent a substance having an active radical which is capable of causing chemical or physiochemical bond and each of Japanese patent publication Nos. 12,518 of 1963, 4,717 of 1973 and 13,456 of 1974 discloses to utilize phase separation by an electrostatic interaction. In these improved methods, however, the steps for making microcapsules are complicated as well as in the so-called "complex coacervation" method. Those methods also involve a disadvantage that the capsule walls are often cracked in the step of drying. This may be considered due to the fact that the microcapsule walls contain water soluble components having opposite net electrical charges.
U.S. Pat. No. 4,001,140 discloses to make microcapsules having aminoaldehyde resin capsule walls by carrying out polycondensation of urea formaldehyde in the presence of ethylene-maleic anhydride copolymer, methyl vinyl ether-maleic anhydride copolymer or polyacrylic acid. However, microcapsule wall films produced utilizing these monomers do not have sufficient heat resistance. In addition this technique is disadvantageous in that when colorless chromogenic materials are used as the core material the colorless chromogenic materials have a color developed. UK patent application GB No. 2018711 A discloses a method of forming microcapsules in which urea and formaldehyde are polymerized around oily droplets in the presence of carboxylated polyvinyl alcohol to form urea-formaldehyde resin microcapsule walls. Although microcapsule wall films produced in this manner has a relatively good heat resistance, their moisture resistance is remarkably decreased. In addition this method is also disadvantageous in that the coloring by colorless chromogenic material used as capsule core material cannot be prevented.
The principal object of the invention is to provide a novel and improved method for making microcapsules each having a capsule wall of an aminoaldehyde resin in which deposition of an aminoaldehyde polycondensation resin on the surface of a capsule core material is effectively carried out.
Another object of the invention is to provide a novel and improved method for making microcapsules having sufficient moisture resistance and sufficient heat resistance.
A further object of the invention is to provide an improved method for making microcapsules which are especially useful for the production of pressure sensitive copying paper.
The other objects and advantages of the invention will become apparent from the following detailed description.